The use of water-based varnishes, lacquers or paint compositions, also called waterborne paints, have been proven for the preparation of printed products such as labels, job printing and folded boxes. When processing in integrated coaters or coating units of sheet fed offset printing machines, water-based paints have been successfully established since the late 70th as so-called overprint varnishes (OPV) for protection of oil-based printing ink.
The basic requirement for the applicability of a paint composition as overprint varnish is, that the paint composition coats the previously wet, oil-based printing ink, which has been applied on a substrate, and forms a protective film thereon.
A classical waterborne paint has generally one or more of the following functions:                protection of the printing ink against abrasion;        achievement of different gloss-levels from matte to high-gloss;        achievement of different slip behaviors from dull to smooth; and        sealing of the surface; the used paints are odorless and tasteless after drying.        
Moreover it is desirable that further functions are met, such as anti-sealing against a packaging film, a temporary barrier against water or grease, or similar functions.
Finally, the water-borne paint compositions should also fulfill certain requirements in view of their processability. For instance, machine runability should be guaranteed, i.e., at usual printing rates of from 10 000 to 15 000 sheets/hr, and up to 18 000 sheets/hr during sheet fed offset printing, the paint has to be uniformly applied and dried to obtain a film. The throughput time through the dryer is usually significantly less than 0.5 seconds. The dryers are generally a combination of an IR radiator and a hot air blower. The temperature within the dryer is usually between 60 and 80° C., if possible at 80° C., and later in a stack at 30 to 40° C. When drying, at least parts of water and optionally other materials, such as e.g. ammonia, are deprived from the paint and the binder particles of the paint approach to such a level that a paint film develops. All these requirements should also apply to a new paint composition.
In order to be applied as a continuous overprint varnish film on a substrate, the waterborne paint composition has to wet the printed substrate, i.e., the printing ink, and otherwise also the substrate as such. On areas without printing ink, the paint has merely to form a continuous film on the substrate, for instance on a paperboard surface, i.e. a film formation takes place.
The water-based paint is wet-applied. e.g., with 3.5-5 g/m2. With a solids content of the water-based paint composition of 30-45%, a dry film with about 1 to 2.25 g/m2 remains on the substrate after drying, depending on the formulation of paint. Then the dry film has to develop an appropriate profile of characteristics, e.g. in order to ensure protection against abrasion. The dry film should also have a certain resistance against humidity and should not be dissolved too quickly under the influence of humidity or moisture and then probably tacks.
The reason is that during the printing process, moisture or humidity will be absorbed (in offset printing, the application of printing ink is controlled by damping water). When bonding any substrates with glue in a later process step, for instance in the production of folded boxes, the paint must not be dissolved again. Thus, humidity may play a role when filling or packing a bulk material, e.g. deep-frozen food, or during a transport at elevated temperatures at, e.g., 70° C., and at a corresponding relative humidity (transport in trucks through hot and humid countries).
Thus, the paint composition is usually selected in a manner so that the paint can form a film at common temperatures of from 60 to 80° C. in a dryer (in a time period of less than 0.5 seconds) and the paint film can later develop a barrier against water. That is, the minimum film (forming) temperature (MFT) of the used paints is generally less than 80° C., typically between 1° C. and 60° C.
In most cases, a conventional paint composition contains as the main component a polymer dispersion and often additionally a polymer resin. The polymer resin has been often neutralized with ammonia, and there are various types of resins having different acid numbers, molecular weight distributions, and glass transition temperatures, which might be used. Numerous different dispersions having various suitable glass transition temperatures (TG) have been used.
The minimum film temperature of a standard paint is at +1° C. to +60° C., preferably at 30 to 50° C. In this range of MFT, a sufficient film formation will be achieved, while the paint forms a continuous film.
Due to the above mentioned profile of characteristics and applications, such paints are also designated as graphic overprint varnishes (overprint varnish, OPV). Moreover, apart from the mentioned conventional printing processes, water-based paint systems have sometimes also advantageously been used in digital printing, for instance in inkjet printing, both as overprint varnish and primer (base coat).
However, conventional waterborne paints have the disadvantage that printing on the paints by inkjet printing involves considerable difficulties. The paint film is formed in such a manner that when printing on the paint film with water-based inkjet inks, the water of the ink cannot be absorbed with a sufficient speed, and even water of the ink cannot evaporate with a sufficient speed so that a wet coverage of ink remains on the paint film. As a result often no adherence of the inkjet ink on the paint film is achieved within an adequate time period. Drying times of less than 0.3 seconds are required for industrial applications because after this process time, the inkjet print has to be resistant to mechanical forces. However, if the ink is not totally dry or has not been absorbed by the paint film when exposed to mechanical forces, the printed design or image may “blur” and can be altered beyond all recognition. Codes, such as bar-codes or QR-Codes, often become illegible or unreadable. Therefore such prints are applied on areas where previously the treatment with paint has been omitted. In this case, the ink is printed on a surface of the substrate being free of paint and can be absorbed. However, the omission of areas of the substrate's surface when applying paint involves additional efforts for the process.
Attempts have been made in the past for adapting a substrate, such as paper or paperboard, to inkjet printing. This can occur for instance already during the production by treatment of the paper- or paperboard materials by means of so-called sizing agents. As a result, a surface having porosity can be obtained which is conducive for inkjet printing. WO 2011/093896 A1 discloses e.g. a paper with a surface treatment for inkjet printing, wherein the paper has been treated with a sizing agent comprising non-film-forming polymer latex and a metal salt. However, on the one hand, a non-film-forming material has been used, making the application as an overprint varnish unsuitable, and, on the other hand, a quite high portion of from 40 to 60 wt.-% of a metal salt has been used, making the composition unsuitable for a use in direct contact with e.g. food or pharmaceuticals because the high portion of metal salt affects food and pharmaceuticals in their material composition and in their taste. In contact with humidity, such salts can be leached out. Due to their composition such formulations—from a technical point of view—are not applicable inside a coater in the offset printing process.